Host processor systems may store and retrieve data using storage devices containing a plurality of host interface units (host adapters), disk drives, and disk interface units (disk adapters). Such storage devices are provided, for example, by EMC Corporation of Hopkinton, Mass. and disclosed in U.S. Pat. No. 5,206,939 to Yanai et al., U.S. Pat. No. 5,778,394 to Galtzur et al., U.S. Pat. No. 5,845,147 to Vishlitzky et al., and U.S. Pat. No. 5,857,208 to Ofek, which are incorporated herein by reference. The host systems access the storage device through a plurality of channels provided therewith. Host systems provide data and access control information through the channels of the storage device and the storage device provides data to the host systems also through the channels. The host systems do not address the disk drives of the storage device directly, but rather, access what appears to the host systems as a plurality of logical volumes. Different sections of the logical volumes may or may not correspond to the actual disk drives.
An individual storage system may contain multiple tiers of storage, each of which may have different performance characteristics and operating costs. For example, one tier may be comprised of Flash drives, another of Fibre Channel drives and another of SATA drives. The character of the storage request access loads that these each drive type is best suited for may vary greatly. In order to efficiently exploit the different capabilities provided by these drive types, the drive type to which a each region of logical storage is mapped would be selected so that the resulting host access workload for the drives would effectively exploit that drives capabilities. For example, SATA drives may handle workloads with low access density and tolerance for higher response times, while Flash drives may handle workloads with high access density and low response time requirements. Fibre Channel drives may handle workloads that do not fall into either of these extremes. The flash drive may be the most expensive storage, the SATA drives the least expensive storage, and the Fibre Channel drives may have a cost that is in between the other two types of drives.
It has been found that a system that uses multiple tiers of data storage can be effective by storing more frequently used data in relatively faster and more expensive tiers while storing less frequently used data in relatively slower and less expensive tiers. See, for example, U.S. patent application Ser. No. 13/135,265 titled: “LOCATION OF DATA AMONG STORAGE TIERS” filed on Jun. 30, 2011, which is incorporated by reference herein. However, in some cases, there may not a sufficient number of different types of storage available having different speeds and corresponding cost. For example, it may be desirable to have three tiers of data storage, but only SATA and flash drives are available while Fibre Channel drives are not available.
Accordingly, it is desirable to provide a system that uses a particular number of separate and distinguishable storage tiers while having less than the particular number of different types of data storage.